AMATEUR RADIO
BY GARRY CRATT, VK2YBX
Build this simple turnstile antenna
for weather satellite reception
Interested in listening to signals from the
polar-orbiting or geostationary weather
satellites? This simple antenna can be built
for -a few dollars yet will give good results.
Most amateur operators appreciate
that, for ground based communications , vertically polarised antennas
with a low angle of radiation perform
best. By stacking dipoles vertically,
the radiation pattern is compressed
so that the usable "gain" is radiated
towards the horizon , thus theoretically encompassing any receiving stations located on the ground.
Conversely, for airborne communications, an antenna having a higher
angle ofradiation is desired, since the
signal must be radiated upwards instead of hugging the ground. The same
applies for the reception of weather
satellite signals.
The easiest weather satellite signals that can be received using simple
equipment are those radiated by the
polar orbiting APT satellites. These
satellites broadcast circularly polarised signals located around 137MHz
and have typical output powers of
DIPOLE 2
/
FEEOPDINT
'-DI
)J4
PHASING LINE
Fig.1: basic layout of a turnstile
antenna (groundplane not shown). It
consists of two 1/2-wavelength dipoles
mounted at 90° and fed 90° out of
phase. Note the 1/4-wavelength
phasing line used to feed the second
dipole.
about 5W or so. Due to their polar
orbit, they pass quite close to the
Earth's surface at heights ranging from
110-BOOkm.
The signals received on Earth from
these satellites are much stronger than
those received from geostationary satellites, which orbit at 37,000km. In
any case, the optimum antenna for
the reception of weather satellite signals is one which combines circular
polarisation and a high angle of radiation.
The turnstile antenna
This photograph shows the general construction of the turnstile antenna. The
reflector was made from small mesh chicken wire attached to a wooden
baseplate, while the dipoles consist of threaded steel rods attached to a wooden
mast. A right-angle metal bracket is used to secure the mast to baseplate.
Although a simple groundplane
antenna having a 1/4-wavelength radiator can be used in applications
requiring a high angle of radiation, a
far more useful antenna is the "turnstile" - so named because of its resemblance to a supermarket turnstile.
A turnstile antenna consists of two
1/2-wavelength dipoles mounted at
goo and fed goo out of phase. When
mounted above a suitabl e reflector as
NOVEMBER 1991
53
This close-up view shows how the dipole elements & the phasing harness are
fitted to the mast. The threaded rod dipoles make it easy to attach the phasing
harness using solder lugs & nuts.
shown in one of the photographs, the
radiation pattern fulfils both our requirements (see Fig.2).
The response of the antennc!, directly above th e dipoles is circularly
polarised, while at angles between
45° and 0°, the response is elliptically
polarised. In this regard, the reflector
spacing is an important consideration, as it determines both the radiation pattern and the feed point impedance of the antenna.
Reflector spacing
As can be seen from Fig.3, if the
reflector spacing is increased from
0.22 to 0.37 of a wavelength, the vertical response is reduced while the response at lower angles (30-50°) is improved. A further consequence of this
reflector spacing is that the characteristic impedance of each dipole is very
close to 100 ohms.
In order to feed the two dipoles out
of phase, they must be connected via
a 1/ 4-wave phasing harness. Fig.1
shows this arrangement. As with other
phasing harnesses, the physical length
of the harness is reduced below a 1/4wavelength according to the velocity
factor of the coaxial cable used.
By selecting a 0.37 wavelength reflector spacing, this arrangement can
be fed with standard 50-ohm coaxial
cable, thus greatly simplifying the construction compared with other reflector spacings requiring a matching section.
In practice, the antenna response is
also improved for receiving polar orbiting satellites.
The accomp anying photographs
show the general construction of the
antenna. The reflector can be made
from small mesh chicken wire or phosphor-bronze m esh, while the dipoles
can be made from aluminium tubing
or steel or brass rods. To protect it
1
Fig.4: this diagram shows the dimensions required for a 1/4wavelength phasing line.
SILICON CHIP
Fig.3: increasing the reflector spacing
of a turnstile antenna to 0.37
wavelength reduces the vertical
response but improves the response at
lower angles (30-50°). This makes the
antenna suitable for satellites passing
at relatively low angles but still gives
useful pickup for satellites passing
almost directly overhead & for
geostationary satellites.
Construction
260mm
54
Fig.2: shown at top is the typical
radiation pattern from a groundplane
antenna, while immediately above is
the pattern for a turnstile antenna
with a reflector spacing of 0.22
wavelength. Note the dramatically
improved vertical response of the
turnstile antenna.
from the weather, the entire antenna
should be sealed using "Estapol" or
some other plastic paint/sealant.
As constructed, the antenna has a
modest gain of around 6dB , which is
quite sufficient for the reception of
polar orbiting weather satellites. Also,
there is no specific requirement for
the antenna to be elevated to any great
degree above the ground, so long as
the "view" is largely unobstructed
from about 20° above the horizontal.
We built the prototype out of scraps
of "chipboard" and softwood planks.
However, for a long-lasting antenna,
chipboard is no good as it deteriorates
rapidly in wet weather. If you want it
to last, use good quality timber and
These photographs show the results that can be obtained by connecting the
turnstile antenna to suitable receiving equipment. All four images are from
GMS-4 but you can also tune into the polar-orbiting satellites.
prime and paint it for weather resistance.
The main mast, which holds the
dipole elements in place, is centrally
located on the baseboard by a right
angle bracket, available from most
hardware stores. The dipole elements
are made from zinc-plated threaded
steel rods and are connected to the
coaxial cable using solder lugs.
Mounting the dipoles
The dipole elements are mounted
by drilling horizontally through the
mast at 10mm intervals. The elements
are then mounted by passing them
through the timber and securing them
using steel nuts and washers.
Another advantage of using this
method is that the 1/4-wavelength
phasing section can be pre-fabricated
on the workbench. We used heatshrink
tubing to seal the jacket of the coaxial
phasing section. As the fe ed point impedance of the dipoles at 0.37 wavelength reflector spacing is about 100
ohms, the phasing line must be made
from 75-ohm coaxial cable. Fig.4
shows the dimensions of this part of
the antenna.
The coax feed can be standard 50ohm type and solder lugs are recommended for the connections to the
dipoles. The reflector uses two overlapping layers of small diameter
chicken wire, available from most
hardware stores (as is- the threaded
rod). As this wire is only available in
900mm wide strips, a total length of
2.6 metres is required to cover all of
the base plate.
We used galvanised staples to secure the wire to the baseboard and
overlapped the screen to give the fullsized reflector dimensions.
Perhaps this simple antenna, which
can be used with an inexpensive scanning receiver or a modified 2-metre
receiver, will encourage readers to further explore the world of weather satellite reception. Just to whet your appetite, we have included several photos of full-screen video pictures, obtained from the more sophisticated
GMS weather satellites, just to show
the remarkable results that are possible.
Of course, you'll need some extra
equipment in addition to your scanning receiver to obtain pictures like
these , including a computer interface
and, possibly, a video printer. We'll
have more to say about this in a future
article. In the meantime, have fun tuning into those satellite signals.
Further reading
(1). Australian Electronics Monthly,
July 1986.
(2). The ARRL Antenna Book.
(3). Weather Satellite Handbook, by
Ralph E. Taggart.
SC
NOVEMBER 1991
55
